Source : Webster's Revised Unabridged Dictionary (1913)

Antithetic \An`ti*thet"ic\, Antithetical \An`ti*thet"ic*al\, a.
   [Gr. ?.]
   Pertaining to antithesis, or opposition of words and
   sentiments; containing, or of the nature of, antithesis;
   contrasted.

Source : WordNet®

antithetic
     adj : sharply contrasted in character or purpose; "practices
           entirely antithetical to her professed beliefs"; "hope
           is antithetic to despair" [syn: {antithetical}]